Rechargeable cells are now in widespread use in many applications. Electronic and, particularly portable electronic, devices ranging from cellular telephone handsets to digital cameras rely heavily upon high-quality rechargeable cells.
In relation to an electronic device having, for example, wireless communications capabilities, the electronic device is typically fitted with an internal charging system to charge a main battery also fitted in the electronic device. The charging system has a pass device designed to be coupled between an external power transformer (hereinafter referred to as an “adapter”) and the battery. However, the widespread availability of third party, and sometimes “pirate”, adapters causes safety concerns for manufacturers of the electronic devices due to the sometimes incompatible output of the adapters. One of these concerns relates to power dissipated by the pass device where the adapter is supplying more power than the power rating of the pass device. Under such conditions, the pass device can be damaged, fail completely, or at the very least the useful lifetime of the pass device can be curtailed; the battery may even become dangerous.
Typically, adapters are designed either for coupling to a wall-mounted power outlet or a cigarette/cigar lighter in a vehicle, such as an automobile. Further, modern adapters are expected to be useable in a number of different countries having differing mains voltage supplies. Therefore, due to these performance demands, some manufacturers of adapters have chosen design alternatives to ensure that only genuine Original Equipment Manufacturer (OEM) adapters are used with the electronic devices, thereby attempting to obviate or at least mitigate against potential harm to charging systems, batteries and/or users. In this respect, manufacturers have designed adapters having low regulated output voltages, and have tried to encourage use of such adapters by providing the adapters with bespoke connector arrangements to couple an adapter to the electronic device. However, whilst such arrangements reduce connection flexibility in a positive way to encourage use of safe OEM adapters, the arrangements increase cost of the electronic device and the handset.
Another approach employs external protective measures. For example, the electronic device can be fitted with a fuse. However, the provision of the fuse increases the cost of the electronic device and cannot be re-used once the fuse has melted or otherwise become disabled and hence the charging system is disabled. Another measure is to provide the charging system with a temperature shutdown circuit, for example a circuit comprising an internal pass device protected by a thermal regulation loop that also maximises charge rate of the charging system, as described in U.S. Pat. No. 6,521,118. However, such a solution is unattractive for the following reasons. Firstly, the circuit describes in U.S. Pat. No. 6,521,118 is not designed to operate with charging systems that employ a pass device external to the adapter. Secondly, the circuit of U.S. Pat. No. 6,521,118 would increase the cost of the electronic device, because it cannot be integrated into a larger power management chip due to the high heat dissipation of the circuit. Consequently, the chip count in the electronic device increases, making the circuit economically incompatible with a low cost charging arrangement. Further, additional internal power dissipation of the circuit is incompatible with disposal of the circuit in a fully integrated power management circuit.
Another technique for coping with excessive power dissipation by the pass device is to use pass devices with higher power dissipation capability. However, higher power rated pass devices require a larger physical package than their lower power rated counterparts and so occupy more circuit board space; they also increase the cost of the electronic device and so are an undesirable solution.
Another known charging system is disclosed in U.S. Pat. No. 6,144,187 and uses an analogue multiplier to calculate an AC adapter input power. A control loop limits the power supplied by the AC adapter, but is more expensive to produce than linear charging systems for electronic devices. Consequently, the above described circuit does not provide protection for pass devices in linear charge applications. Additionally, this circuit limits the amount of power available to charge the battery.